The present invention relates to stock, a method for preparing it, the use of the stock as a raw material for producing printing paper, especially newsprint, and a printing paper. The stock produced in accordance with the method of the present invention can be used as a raw material for producing different papers, such as SC paper (supercalendered) comprising both offset and gravure grades, coated paper having a low grammage or LWC paper (light weight coated) comprising both offset and gravure grades, and newsprint or corresponding printing papers. Newsprint also comprises other grades of paper than those used in newspapers, e.g. catalogue papers and gravure papers.
A known method for producing mechanical pulp is presented in patent publication U.S. Pat. No. 5,145,010, corresponding to international application WO 8906717 and Swedish patent publication SE 459924. The method comprises the following phases:
impregnating softwood chips with water and chemicals
primary refining of the treated chips
fractionating the refined softwood pulp into accept and reject stock portions, whereby the reject portion comprises 15-35% of the refined stock
refining of the reject stock portion in two steps, whereby the stock consistency in the first step is approximately 20-35% and in the third step approximately 5%, and
the above-mentioned stock is fractionated to form an accept stock portion and a reject stock portion. The fractionation is carried out with a screen.
A known method for producing mechanical pulp is presented in patent publication U.S. Pat. No. 4,938,843. The process involves the production of chemi-thermomechanical stock. The chips impregnated with chemicals and treated with heat are refined to a freeness value of 100-700 ml CSF, usually in a two-phase refining process and screened to form a first accept stock portion and a first reject stock portion, so that at least 30% of the stock goes into the reject stock portion. The consistency of the stock during the screening is approximately 2%. The first accept stock portion is screened for a second time, whereby a second accept stock portion and a second reject stock portion are formed. The first and the second reject stock portions are combined, creating a long-fibre fraction with a freeness value of 200-750 ml CSF, which can be used separately to produce coarse-fibred products, for example cardboard, or it can be further refined and returned to the first screening.
A known method is the method for producing stock described in the introductory part of patent claim 1 of the present application, in which method the process is begun with two-phase refining. The chips are fed into the first refiner, from which they are fed into the second refiner after the first refining is complete. After the second refiner, the freeness value of the stock is about 120 ml CSF. The consistency is typically 50% at the first refiner and 45% at the second refiner. After the first refiner, the measured average fibre length, when using spruce as the raw material, is approximately 1.7 mm, and after the second refiner the average fibre length when using the same raw material, is approximately 1.5 mm. After the second refiner there is a latency chest, in which the fibres are straightened by diluting the consistency to 1-2%. The fibres are treated in the latency chest for one hour. The fibres are conveyed to the first screen, which fractionates the stock into an accept portion and a reject portion. The freeness value of the accept stock portion is about 20 ml CSF. Water is removed from the reject stock portion to obtain a consistency of 45%. The reject stock portion, which constitutes 40-50% of the total stock, is conveyed to the third refiner, from which the reject stock diluted to a consistency of 1%, is transported on to a second screen. Again the stock is fractionated into an accept stock portion and a reject stock portion. The reject stock portion is conveyed, after the removal of water, at a consistency of 45%, to a fourth refiner and after being diluted to a consistency of 1%, on to a third screen. The reject stock portion from this screen is fed again to the fourth refiner. The stock obtained from the process has a freeness value of 30-70 ml CSF. advantageously about 50 ml CSF. The pressure used in the refiners is 350-400 kPa. The process consumes about 3.3 MWh/t of energy (using spruce as the raw material), 0.3 MWh/t of which is used for regulating the consistency so as to be suitable for every stage of the process.
In the process mentioned above, regulating the consistency to a suitable level consumes 9-10% of the total energy used in the process. In the present application, consistency refers to the amount of stock as a percentage by weight in the mixture of pulp and water. The water can be either in vapour or in liquid form.
After the refining, a latency chest, in which the fibres remain, is needed to straighten the fibres. Regulating stock consistency requires the use of suitable equipment, for example presses to press water out and pumps to pump water into the process. This means that the process is prolonged and the equipment for the process is complex. In addition, the problems of the known processes include high energy consumption, a relatively short average fibre length of the obtained stock, and mainly due to this, deficiencies in the tensile strength and tear resistance of the printing paper produced from the stock.
The above-mentioned problems can be reduced by the method of the present invention for producing stock, the stock itself, the use of the stock in producing printing paper and the printing paper itself. The method of producing stock in accordance with the present invention is characterised in that the stock is screened at a consistency of no less than 10%. The stock produced in accordance with the present invention is characterised in that at least 40% by weight of the fibres do not pass through a Bauer-McNett screen with a mesh size of 28. The printing paper produced in accordance with the present invention is characterised in that it has been made of stock that has been produced by the method in accordance with the present invention, or stock that has the same fibre distribution as the stock produced by the method of the present invention.
In the method of producing stock in accordance with the present invention, the stock is screened at a high consistency, whereby it is not necessary to change the consistency to suit each refining step between the refiner and the screen, but the refining and screening can be done at essentially the same consistency. The amount of energy that is consumed for pumping water into the process and pressing it out of the stock can thus be saved. When using the new screening method, there is no need for pumps to pump water, presses to remove water or a latency chest between the refining and screening steps of the process, whereby the process becomes simpler.
When using the new type of screen that screens stock at a high consistency, the quality of the stock improves because the screen efficiently separates the coarser fibres that need further refining into a reject fraction, and flexible, long fibres into an accept portion. In this way the printing paper produced from the long-fibre stock has good formation. The resin remains in the fines because, due to the high consistency, it cannot spread onto the surface of the fibres.
The screen is simple and does not contain moving parts and therefore its manufacture and maintenance costs are low. The size of the screen is small because the screening process is carried out at a high speed. Due to its small size, the manufacturing costs of the screen are low. The screen can utilise the steam produced in the refiner as the screening force, as a result of which no separate sources of power are necessarily required.
In addition, thanks to the new refining process, the energy consumption is lower than in the known methods which aim at the same freeness value. In this patent application, freeness refers to Canadian Standard Freeness, the unit of which is ml CSF. Freeness can be used to indicate the refining degree of the stock. According to the literature, the following correlation exists between the freeness and the total specific area of the fibre:
A=xe2x88x923.03 ln(CSF)+21.3
where
A=the total specific area of the stock (unit m2/g)
According to the formula mentioned above, the total specific area of the stock increases as the freeness decreases, that is, the freeness gives a clear indication of the refining degree because, as the proportion of fines grows, the specific area of the fibres increases.
In accordance with the present invention, the process produces mechanical stock in which the proportional amount of long fibres is high. The term mechanical stock is used in this application to indicate stock produced by refining wood raw material, such as chips. In connection with the refining, the wood raw material and/or stock is heat-treated in order to soften the wood raw material, in which case the process is that of producing thermomechanical pulp. The wood raw material may have also been treated with chemicals before being refined, in which case the process is that of producing chemi-thermomechanical pulp.
Because the average fibre length of stock produced in accordance with the new method is longer, the tensile strength and tear resistance of printing paper produced from this stock consisting of primary fibres are also improved. The proportion of long fibres in the stock is higher than in stocks produced by the known methods, such as the stock described above as the product of the process closest to,the state of the art. In the new method, the proportion of short fibres remains more or less the same as in the known methods, but the proportion of fibres of medium length decreases in the stock produced by the new method.
The stock can be used to manufacture printing paper, for example, newsprint, the grammage of which can be lower than that currently used in newsprint, while the properties of the paper still remain good. The stock can be used to manufacture newsprint, the grammage of which can be 30-40 g/m2, measured at a temperature of 23xc2x0 C. and at a relative humidity of 50%.
Because paper with good strength properties can be obtained as the end product when utilising the manufacturing method presented by this invention, more fillers can be used to replace fibre than at present. For supercalendered paper, the filler content to be used can be approximately 30%, and for newsprint 7-15%, advantageously approximately 10%. It is noteworthy that the stock can be used to manufacture printing paper, the grammage of which can be lower than that of the printing paper normally used at present, and at the same time the filler content can be increased, even though fillers reduce the strength of the paper. Fillers are cheaper than fibre raw material and improve the light scattering coefficient and opacity of the paper.
The tree species that have been presented in this application as suitable raw materials are spruce (Picea abies), pine (Pinus sylvestris) and southern pine (genus Pinus, several different species). It is also feasible that the stock made of wood raw material may contain stock obtained from at least two different tree species and/or stock prepared in at least two different ways, which at a suitable phase of preparation are mixed with each other. For example in supercalendered paper and in low-grammage coated papers, chemical pulp obtained by chemical cooking is generally one of the raw materials used, whereas it is not used in newsprint. The amount of chemical pulp in supercalendered paper is usually 10-20%, and in low-grammage coated papers 20-50% of the pulp composition. The pulp composition refers to the total fibre stock used for the manufacture of paper.
The properties required for newsprint grade, which is one important use of the stock presented in this patent application, are runnability, printability and appearance. What is meant by good runnability is that the paper can be conveyed through a printing machine without breaks in the web. Paper properties affecting the runnability of paper include tear resistance, formation, tensile strength, elongation and variation in grammage.
Printability means the ability of the paper to receive the print and to retain it. Printing ink must not come off when rubbed, transfer from one sheet to another or show through the paper. Paper properties affecting the printability of paper include, for example, smoothness, absorbency, moisture content, formation, opacity, brightness, porosity and pore size distribution.
The appearance of the paper can be judged by its optical properties, such as brightness, whiteness, purity and opacity.
The basic idea of the stock preparation method presented in this invention is to use a simple and energy saving process to manufacture stock in which there is a high relative proportion of long fibres. The average fibre length obtained by utilising the method is approximately 10% longer than in the prior art method. At the first stage of refining the wood raw material is refined at a high temperature, advantageously at a temperature of 165-175xc2x0 C., and at a superatmospheric pressure of over 400 kPa, advantageously at a superatmospheric pressure of 600-700 kpa, for only a very short time, as a result of which the stock remains quite coarse after the first stage of refining. What is meant by superatmospheric pressure is the pressure in comparison with normal atmospheric pressure. The average retention time of the raw material to be fed in the high pressure refiner is only 5-10 seconds. The temperature at which refining takes place is determined by the pressure of the saturated vapour.
After the first stage of refining, the stock is screened so as to produce a first accept stock portion and a first reject stock portion. When the stock has been screened into a first accept stock portion and a first reject stock portion, there are different possible procedures for continuing the process, such as
1-step processing of the first reject stock portion, in which the reject stock portion is refined and screened in one step. Accept stock portions are taken out of the process after each stage of screening and/or accept stock portions are re-screened, or
2-step processing of the first reject stock portion, in which the reject stock portion is refined and screened in two steps. The accept stock portions are taken out of the process after each stage of screening and/or accept stock portions are re-screened, or
3-step processing of the first reject stock portion, in which the reject stock is refined and screened in three steps and the accept stock portions are taken out of the process after each screening stage, or
forward-connected 2- or 3-step processing of reject stock, which means the processing of the reject stock first in two or three steps and the removal of the accept stocks after each screening stage, and thereafter the refining of the last reject stock portion, for example, in a low-consistency refiner and removal from the process of the whole stock processed in the low-consistency refiner.
In the above-mentioned alternatives, one step consists of a successive refiner and screen. Further on, the above-mentioned embodiments are described in detail. The accept stock portions obtained at different stages of the process are combined and mixed, possibly bleached, and utilised as a raw material for making paper in a paper machine. The machinery for preparing the stock may consist of several parallel processing lines, from which all the obtained accept stock portions are combined.
The first stage of refining, the so-called primary refining, is advantageously conducted in a one-stage process. There may, however, be several parallel refiners at the same stage. A refiner may be a conical or a disc refiner, advantageously a conical refiner. A conical refiner produces longer pulp fibres than a disc refiner. After the first stage of refining, the pulp is screened into a first accept stock portion and a first reject stock portion. Screening is conducted at a high consistency, of not less than 10%. More advantageously, screening is conducted at a consistency of not less than 20%, and most advantageously at a consistency of not less than 40%. However, the consistency of the material being screened may not be more than 90%, more advantageously not more than 80% and most advantageously not more than 60%.
The stock is fed into a refiner either by means of a separate power source, for example, compressed air, or by utilising the outlet pressure of a refiner, which pressure at the first stage of refining is over 400 kPa, advantageously 600-700 kPa, and at the subsequent stages after the first stage of refining, either over 400 kPa, advantageously 600-700 kPa, or not more than 400 kPa, advantageously 300-400 kPa. The stock leaving the refiner is a mixture of steam and fibres with a consistency of 40-60%. The water is in the form of steam.
The process results in a stock with a freeness value of 30-70 ml CSF. Stock of this kind is suitable for making printing papers, and because the stock also contains very long fibres, the paper will possess good strength properties. The paper will also have good printing properties.
The fibre distribution of a ready-made stock measured according to the Bauer McNett characterisation is as follows:
40-50% of the fibres do not pass through screens of 16 mesh and 28 mesh,
15-20% of the fibres pass through screens of 16 mesh and 28 mesh, but do not pass through screens of 48 mesh and 200 mesh, and
35-40% of the fibres pass through screens of 48 mesh and 200 mesh, in other words these fibres pass through all the screens used (xe2x88x92200 mesh).
The average fibre length of fibres retained on a screen of 16 mesh is 2.75 mm, the average fibre length of fibres retained on a screen of 28 mesh is 2.0 mm, the average fibre length of fibres retained on a screen of 48 mesh is 1.23 mm and the average fibre length of fibres retained on a screen of 200 mesh is 0.3 mm. (J. Tasman: The Fibre Length of Bauer-McNett Screen Fractions, TAPPI, Vol.55, No.1 (January 1972))
The stock thus obtained contains 40-50% of fibres with an average fibre length of over 2.0 mm, 15-20% of fibres with an average fibre length of over 0.35 mm, and 35-40% of fibres with an average length of less than 0.35 mm.